This invention relates generally to a way to improve the sealing of prismatic-shaped battery cells, and more particularly to the use of barrier layers made from extremely hydrophobic materials along the borders or edges of stacked planar battery cells as a way to improve corrosion resistance and reduced vapor permeability through the regions where adjacent cells are joined.
Lithium-based batteries are being used in automotive and related transportation applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes such batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. The flow of electric current to and from the individual cells (i.e., a single electrochemical unit) is such that when several such cells are combined into successively larger assemblies (such as modules and packs), the current or voltage can be increased to generate the desired power output. In the present context, larger module and pack assemblies are made up of one or more cells joined in series (for increased voltage), parallel (for increased current) or both, and may include additional structure to ensure proper installation and operation of these cells. One common vehicular form of the battery pack is known as a power battery, while another is known as an energy battery.
In the power battery pack variant, the individual cells that make up a battery pack are configured as prismatic (i.e., rectangular) cans that define a rigid outer housing known as a cell case. In the energy battery pack variant, the individual cells are housed in a thinner, flexible prismatic pouch. In situations where the pouch forms a containment structure for the electrically active internal components, the present inventors have joined the outer periphery of adjacently-facing planar pouch surfaces together with adhesive-based heat seals. The present inventors have determined that such heat seals (which may be made up of one or more layers of polypropylene, nylon, polyethylene and adhesive) do not provide protection against edgewise inward oxygen and water vapor permeation or edgewise outward electrolyte solvent vapor escape, and that either such phenomena can—if left unchecked—lead to premature cell failure or performance degradation. Attempts to provide additional features to reduce susceptibility to such leakage are unavailing, as they are complex and expensive.